1. Field of the Invention
The invention relates generally to displacement reactions for synthesis of carbon nanotubes and metal encapsulated within a carbon lattice structure.
2. Description of Related Art
Pure carbon has many allotropes, such as: diamond; graphite; fullerenes; and nanotubes, each being stable in different temperature and pressure ranges. Fullerenes are a family of closed caged molecules formed entirely of carbon in the sp2-hybridized state and constitute a third form of carbon after diamond and graphite. These spherical, cavity containing molecules and their allotropes have been found to possess remarkable properties, and the most stable one known as buckminsterfullerene or C60 has been widely investigated.
In 1991 Sumio Iijima synthesized, through the use of an arc discharge method, new carbon structures in the form of needle-like tubes or rolled-up graphite sheets with multiple concentric cylindrical shells of hexagonally bonded carbon atoms. These extended fullerene tube structures have been called carbon nanotubes, more specifically multi-walled nanotubes (MWNTs), having a thickness of several carbon atom layers and typical outside diameters from a few to several tens of nanometers. A variation of the arc discharge method, using two graphite rods of different diameters, has also been reported to produce MWNTs. [Ebbeson, T. W., et al., Nature, Vol. 358:220-222 (1992)]. MWNTs can also be synthesized by catalytic decomposition of hydrocarbons on metal surfaces. [Rodriguez, N. M., et al., Langmuir, Vol., 11:3862-3866 (1995)].
In 1993 it was discovered that the use of transition metal catalysts during arc discharge produced single walled nanotubes (SWNTs) [Bethune, D. S., et al., Nature, Vol. 363:605-607 (1993), Iijima, S., et al., Nature, Vol. 363:603-605 (1993)]. Kiang, C-H. et al., described the synthesis of SWNTs with a metal catalyst [Kiang, C.-H., et al., J. Phys. Chem. Solids, Vol. 57:35-39 (1995); Kiang, C.-H., et al., J. Phys. Chem., Vol. 98: 6612-6618 (1994); Kiang, C.-H., et al., Carbon, Vol. 33:903-914 (1995); Kiang, C.-H., et al., Chem. Phys. Left., Vol. 259:41-47 (1996)].
SWNTs have been generated by arc-evaporation in the presence of a cobalt catalyst [Bethune, D. S., et al., Nature, Vol. 363:605-607 (1993) and Ijima, S., et al., Nature, Vol. 363:603-605 (1993)]. A recent modification of this arc-evaporation method has been reported to enable SWNT synthesis in larger yields [Journet, C., et al., Nature, Vol. 388:756-758 (1997)]. SWNTs are also produced by laser-vaporization of a graphite, cobalt, and nickel mixture at 1200xc2x0 C. [Guo, T., et al., Chem. Phys. Lett., Vol. 243:49-54 (1995)]. This method was optimized to reportedly give 70% yield [Thess, A., et al., Science, Vol. 273:483-487 (1996)]. More recently, SWNTs were synthesized by the Thess et al., method without the assistance of oven heating [Maser, W. K., et al., Chem. Phys. Left., Vol. 292:587-593 (1998)]. SWNTs are also produced by thinning of MWNTs using CO2 by pyrolysis of the hydrocarbon [Cheng, H. M., et al., Appl. Phys. Left., Vol. 72:3282-3284], and by chemical vapor deposition [Hafner, J. H., et al., Chemical Physics Letters, Vol. 296:195-202 (1998)].
It has been reported that hydrogen gas can condense inside SWNTs [Dillon, A. C., et al., xe2x80x9cStorage of hydrogen in single-walled carbon nanotubesxe2x80x9d, Nature, Vol. 386:377-379 (1997)], and that elongated crystallites of Ru were encapsulated in SWNTs [Sloan, J., et al., The opening and filling of single walled carbon nanotubes (SWNTs), Chem. Commun., Vol. 3: 347-348 (1998)]. Metal nanoparticles have been reported to be encapsulated in graphite layers by a modified arc evaporation method [Jiao, J., et al., Journal of Applied Physics, Vol. 80:103-108 (1996)]. Nanoparticles suitable for magnetic recording media, synthesized via the arc-evaporation method, have been described in U.S. Pat. No. 5,456,986 to Majetich, et al., and U.S. Pat. No. 5,783,263 to Dravid, et al. U.S. Pat. No. 5,780,101 to Nolan, et al., described a method for producing encapsulated nanoparticles and carbon nanotubes using catalytic dis-proportionation of carbon monoxide.
Nanotubes are superstrong and lightweight and can act as either a conductor or a semiconductor depending on the inner diameter and chirality of the hexagonal carbon lattice along the length of the nanotube. See, Dekker, C., xe2x80x9cCarbon Nanotubes as Molecular Quantum Wiresxe2x80x9d, Physics Today, Vol. 52:22-28 (1999), Ebbeson, T. W., xe2x80x9cCarbon Nanotubesxe2x80x9d, Physics Today, Vol. 49:26-32 (June 1996). Based on their size and weight, nanotubes have novel electrical, optical, magnetic, and thermal properties. See, Han, S., et al., Science, Vol. 277:1287 (1997); Vietze, U., et al., xe2x80x9cZeolite-Dye Microlasersxe2x80x9d, Phys. Rev. Lett., Vol. 81:4628-4631 (1998); Service, R. F., Science, Vol. 281:940-942 (1998); and Heremans, J., et al., xe2x80x9cMagnetoresistance of bismuth nanowire arrays: A possible transition from one-dimensional to three-dimensional localization,xe2x80x9d Phys. Rev. B 58: R10091 (1998).
Some of the many potential applications of nanotubes include: molecular electronics [Tans, S. J., et al., xe2x80x9cRoom-temperature transistor based on a single carbon nanotubexe2x80x9d, Nature, Vol. 393: 49-52 (1998)], hydrogen storage media [Dillon, A. C., et al., xe2x80x9cStorage of hydrogen in single-walled carbon nanotubesxe2x80x9d, Nature, Vol. 386: 377-379 (1997)], and scanning probe microscope tips [Wong, S. S., et at., xe2x80x9cCovalently functionalized nanotubes as nanometer-sized probes in chemistry and biology,xe2x80x9d Nature, Vol. 394: 52-55 (1998)]. Nanotubes can be created with acidic functionality or with basic or hydrophobic functionality, or with biomolecules at the open tip ends. Macro-applications include lightweight, strong wires, batteries, fuel cells, or bulletproof vests. Biological applications include an open-ended nanotube that could inject a few molecules into a specific region of a cell to carry out molecular surgery on nucleic acids [Yakobson, B., et al., xe2x80x9cFullerene Nanotubes: C1,000000 and Beyond,xe2x80x9d American Scientist, Vol. 85:324-337 (1997)]. These applications are but a few of the applications requiring strong, small diameter nanotubes. Also, Co, Fe, and Ni are magnetic metals and are of interest as magnetic data storage media.
Solid-state metathesis (SSM) reactions have been reported to be a rapid route to many solid-state materials including chalcogenides, nitrides, borides, phosphides and intermetallics [Gillan, E. G., et al., Chem. of Mater., Vol. 8:333-343 (1996); U.S. Pat. No. 5,110,768 to Kaner, et al.; and Wiley, J. B., et al., Science, Vol. 255:1093-1097 (1992)]. A solution phase process for the synthesis of Group III-V semiconductor nanocrystals has also been reported [U.S. Pat. No. 5,474,591 to Wells, et al.].
It has been reported that MWNTs, grown in the vapor phase, are being produced on the kilogram scale daily, but little is known about the possibility to further scale-up this method. There is no comparable method for the bulk synthesis of SWNTs [Service, R. G., supra.] or of metal encapsulated within a carbon lattice structure. Furthermore, despite the development of SWNT production, the current cost of purified SWNTs is prohibitive, in the range of $1000 per gram.
The present invention overcomes drawbacks of the foregoing methods to rapidly prepare carbon material, having at least a partially curved structure, and/or encapsulated metal within a carbon structure. By partially curved structure is meant having a non-flat carbon based structure, such as found in nanotubes. An advantage of the present invention is an increase in yield as the reaction is scaled up.
The invention comprises a displacement reaction, preferably a double displacement (solid-state metathesis (SSM)) reaction in which a carbon compound, such as a hydrocarbon, halogenated hydrocarbon, or halogenated carbon compound and a metal compound are metathetically reacted in the presence of a catalyst, using heat to initiate the reaction, which is otherwise exothermic, to provide a highly efficient, inexpensive and readily scalable route to MWNTs, SWNTs, and metal encapsulated within a carbon lattice structure.
In particular the carbon compound preferably has the formula RnXyHz, wherein R is carbon, n is a number from 1 to a million or more, preferably 1 to 1000, preferably still 1 to 100; X is selected from the halide group consisting of fluorine, chlorine, bromine, and iodine, y is 0 to a million or more; H is hydrogen, z is 0 to a million or more and the ratio of y to z represents the degree of halogenation of the hydrocarbon; the metal compound is represented by Mxxe2x80x2Ryxe2x80x2 wherein M is any of the Group 1, 2, or 13 metal ions capable of forming a salt; R is carbon, xxe2x80x2 is any integer, preferably a number from 1 to 3, yxe2x80x2 is a number from 0 to a million or more, preferably 0-3, preferably still, xxe2x80x2=yxe2x80x2=2. Preferably the metal compound is lithium acetylide. he catalyst is preferably a transition metal catalyst, such as CoCl2, NiCl2, or FeCl3 or an organometallic metal. Heat to initiate the exothermic reaction can be provided from any conventional source, for example, a heated wire.
The displacement reaction of the present invention uses inexpensive precursors, requires less preparation, and less expensive equipment than existing methods. The reaction also produces nanoparticles comparable in size to those synthesized by modified arc evaporation. Thermodynamic reaction parameters such as maximum reaction temperature and exothermicity can be altered by changing reactants, (for example, from halogenated carbon compounds to halogenated hydrocarbons or pure hydrocarbons) reaction size, or the addition of inert salts, or combinations thereof. These parameters, in conjunction with the type of catalyst and gas atmosphere used, enables the optimization of nanotube yield. Also, the nanotube yield increases with increasing reaction size.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings.